How do you check whether a book is right?

That time, that place

A Franciscan friar who moved between Oxford and Paris. He read Ibn al-Haytham's optics, arrived through Toledo, and the method he found there shook him.

Why this question

In his day, settling a question meant searching the authorities. What did Aristotle say? What did the Fathers say? Argument came down to a contest of citations.

Bacon thought something was missing. However tight the reasoning, a doubt remains in some corner of the mind — and doing the thing yourself is what removes it.

What was found

He called it experimental science, and his example was the rainbow. Whatever the books say, make one with water droplets and light and measure the angle. He did measure it, arriving at a value near forty-two degrees.

Yet he also believed in alchemy and astrology, and never carried out the experiments he called for on any scale. Naming a method and living by it are not the same thing.